1. Field of the Invention
The present invention relates to a speed change apparatus for a vehicle.
2. Description of Background Art
A dog clutch transmission of the so-called automated manual transmission (AMT) type is known wherein a shift is conducted automatically. This structure makes it easy to restrain the “shallow engagement” of dog teeth which might occur when the clutch disengagement time during a shift is short. The shallow engagement of dog teeth refers to a condition wherein the depth of the mesh of dog teeth is shallower than the depth of the mesh in a normal engagement state of dog teeth. In the shallow engagement canceling mechanism in the conventional example, a wavy profile is set in a shift-up direction of a clutch lifter cam plate. When the clutch lifter cam plate is rotated in the shift-up direction, an engagement/disengagement operation can be instantaneously repeated in the manner of clutch disengagement→engagement→disengagement, whereby shallow engagement can be canceled. When a driving force is acting on the transmission, the shallow engagement of dog teeth is maintained by a frictional force between driving force transmitting surfaces of the dog teeth which abut on each other. When the driving force acting on the transmission is removed, the dog teeth are permitted to move to a normal depth of mesh, whereby the shallow engagement is canceled. See, for example, Japanese Patent Laid-Open No. 2014-199102.
On the other hand, while the conventional example describes a shift in the shift-up direction and the cancellation of the shallow engagement, the example does not describe a shift in a shift-down direction. The transmission in the conventional example adopts a so-called interlocked spindle system wherein both a clutch lever and a transmission master arm are operated by a single shift spindle. Between the shift spindle and the master arm, a force accumulation mechanism is interposed. The force accumulation mechanism operates only in the shift-up direction to accumulate a rotational amount and a load of the shift spindle. This force accumulation mechanism does not operate in the shift-down direction. The shift spindle is so configured that when the shift spindle is rotated in the shift-down direction, a gear shift arm is rotated by an amount corresponding to a circumferential clearance between the gear shift arm and the master arm (in the course of this operation, the clutch is disengaged) and, thereafter, the gear shift arm moves the master arm.
In the structure of the conventional example, at the time of a shift in the shift-down direction, the shift can be performed as follows. When the shift spindle is rotated to a shift-down side target angle and a certain lapse of time is waited for in a condition where the clutch is disengaged and a driving force is not transmitted, the master arm is rotated during the waiting time and gear trains are moved, so that dog teeth are moved into normal positions, whereby the shift can be completed. In an AMT type vehicle, however, the driving force non-transmission period during an automatically performed shift may cause an uncomfortable feeling for the driver. In order to reduce the uncomfortable feeling about the driving force non-transmission, it is necessary to shorten the time for which the clutch is in a disengaged state. If the driving force non-transmission period is shortened, however, the time allowed for operations and movements of a change system and transmission gears is shortened, so that shallow engagement of dog teeth is liable to occur. For example, when “dog abutment” wherein top surfaces of dog teeth abut on each other is generated, a certain length of time may be needed for cancellation (removal) of the dog abutment. If a driving force is exerted immediately after the dog abutment is canceled, driving force transmitting surfaces on the tip side of the dog teeth may contact each other before the dog teeth mesh with each other in a normal depth of mesh, resulting in shallow engagement.
Thus, shallow engagement becomes liable to occur when the driving force non-transmission period during a shift on the shift-down side is shortened. Therefore, cancellation of the shallow engagement becomes a task. In the transmission according to the conventional example, rotation of the shift spindle toward the shift-down side disengages the clutch first, and thereafter rotates the master arm. When the shift spindle is again rotated toward the shift-down side after the shallow engagement for the purpose of canceling the shallow engagement, the clutch is first disengaged and the shallow engagement is thereby canceled. However, when the shift spindle is further rotated in the shift-down direction, a shifting-down operation on the change mechanism is carried out once more. In other words, if the shift spindle is rotated once more in the shift-down direction at the time of canceling the shallow engagement on the shift-down side, a shift-down in an amount corresponding to two gear positions (speeds) would possibly be carried out, depending on the rotational amount of the shift spindle.